I'd be looking for marks in the brass about the size and shape of the balls. Similarly, they should come out with normal chamber cleaning. If there are loose balls inside the receiver, it may be that a loose one worked its way between the bolt lugs and the shelf. Maybe blast the lug recesses with brake cleaner. I use an AR-10 chamber brush on a short rod when cleaning my 308-family chambers and lug recesses. Check the bolt head and its lugs for damage or wear consistent with the balls.

Take a look at the primary extraction ramp on the bolt handle and its contact surface on the receiver as well. Looks for damage or unusual looking wear.